This invention relates to apparatus for encoding digital information into, and decoding it from, a signal capable of being recorded on magnetic tape or other recording medium. It is of particular utility in apparatus for recording and reading digital information in which insensitivity to variations in speed of the recording medium is desired.
In the prior art, variable-ratio pulse width modulation systems which require two transitions per digit are shown in Wolf U.S. Pat. No. 3,720,927 and Spiecens U.S. Pat. No. 3,959,816. In these systems the total time for the two successive transitions defining each digit is constant. Minnesota Mining and Manufacturing Company (3M) makes a variable-ratio system requiring two transitions per digit wherein one transition is of fixed length and the other transition is of variable length, so that the total time, or cell width, of the two transitions is not constant.
Murphy U.S. Pat. No. 4,021,853 shows a phase modulation system, which generally provides one transition per digit but frequently provides only one transition for two digits. Phase modulation systems are notoriously subject to noise and are extremely sensitive to speed variations of the recording medium.